The present invention is directed in general to a pulse generator and more particularly to an integrated circuit pulse generator which may be externally and digitally conditioned for providing output pulses at a frequency within a predetermined range of frequencies corresponding to a specified frequency tolerance.
There are many applications where it is necessary to be able to generate a series of successive pulses at a frequency which is held to extremely tight tolerances. A prime example of such an application is in the area of electric timekeeping devices such as electric clocks and watches. Where electric time pieces are electrically powered by a 60 cycle alternating current, precise timekeeping is not a problem because an accurately controlled 60 cycle electric power waveform is readily available. However, when the time indicating means of such clocks are powered by a battery or some other d.c. source, the problem of providing an accurately controlled waveform for powering the clock motors or other time indication means becomes extremely acute.
One type of battery powered clock which has found wide acceptance in recent years is the quartz clock. Quartz clocks generally include a pulse generator which coverts a d.c. voltage into electric pulses wherein the electric pulses drive either a time indicating mechanism motor or solid state time indicating devices. Indicative of the name by which they are called, the quartz clock pulse generators comprise a fixed frequency oscillator which includes a frequency-determining quartz crystal. The oscillators provide an output waveform having a first frequency which is divided down by pulse generator dividing circuits to a suitable output frequency for driving the clock motors or solid state devices. Because the pulse generator oscillators are crystal controlled, the output waveform frequency is held virtually constant.
Unfortunately, crystals cannot be economically manufactured to oscillate at a precise desired frequency. Variations in free oscillating frequency must be tolerated and compensated for. As compensation, quartz clocks have generally included a variable trimmer capacitor associated with the oscillator circuit for adjusting the oscillator frequency to a nominal value.
While the prior art techniques of providing a constant series of successive pulses at a tightly controlled frequency have been acceptable, there remains substantial room for improvement to that end. Where a variable capacitor is utilized to compensate for off frequency crystal resonance, the compensating procedure must be performed by hand at considerable expense. Also, only a narrow range of compensation is possible, and as a result, the crystals must be manufactured to close tolerances which makes the crystals not only initially expensive in a highly competitive commercial environment, but also requires continuous and expensive quality control acceptance procedures. Lastly, variable capacitors are inherently subject to instabilities with the passing of time and with changes in environmental conditions such as temperature and humidity. As a result, it is not unlikely that periodic readjustment of the variable capacitor will be required over the useful life span of a pulse generator incorporating a crystal controlled oscillator and a variable compensating capacitor.
There currently exists a partial solution to the problems associated with the prior art. It takes the form of an integrated circuit which includes an internal oscillator adapted for connection to an external crystal and internal divider circuits. Associated with the divider circuits is an internal pulse inhibitor which has weighted programming input terminals which cause the pulse inhibitor to inhibit the oscillator pulses and to thereby controllably vary the frequency of the output signal in delta amounts corresponding to a binary progression. The integrated circuit identified as SCL 5419 manufactured by Solid State Scientific, Inc. of Montgomeryville, Pennsylvania, is adapted to be mounted on an insulating substrate having conductive leads deposited thereon for grounding and actuating each input. This integrated circuit eliminates the variable compensating capacitor, and it has been found that the binary weighted pulse inhibit operation allows broader compensation than is obtainable with variable capacitors.
For controllably varying the output signal frequency, drilling through or otherwise removing portions of certain ones of the conductive leads is required. This procedure requires expensive and precise tooling or must be performed by hand.
This integrated circuit however only represents a partial solution. Because the conductive ground leads occupy considerable substrate area, the substrates are too large to be conveniently received within certain small clock housings or watch cases. Also, the possibility always exists that metal burrs or the like resulting from the drilling process may bridge the otherwise open gap in a conductive lead to result in the wrong frequency being produced.
It is therefore a general object of the present invention to provide a new and improved pulse generator.
It is a further object of the present invention to provide a pulse generator which provides an output waveform at a tightly controlled frequency and which is small enough in size to be received within small clock housings or watch cases.
It is a still further object of the present invention to provide a pulse generator which includes a frequency-determining crystal and which may be automatically adjusted to provide an output signal frequency within a predetermined range of frequencies corresponding to a specified frequency tolerance.
It is a still more particular object of the present invention to provide a pulse generator which is predominantly in integrated circuit form and which includes digital adjustment means within the integrated circuit.
It is a still further object of the present invention to provide a system for automatically and externally conditioning an integrated circuit pulse generator to provide an output frequency within a predetermined range of frequencies.
It is a still more particular object of the present invention to provide a signal transforming means for controllably varying by selectable delta amounts a predetermined parameter of an applied signal.